The present invention relates to adjustment of an oscillator. Particularly, but not necessarily, the invention relates to the adjustment of an oven stabilized crystal oscillator in a base station system over a packet switched network for improving the long-term stability of the oven stabilized crystal oscillator.
The base station system in a digital cellular radio system, such as a GSM (Global System for Mobile Communications) network, typically comprises one or more base stations and a base station controller which controls said base stations. Timing and clock signals used in the base station and the base station transmission frequency are typically derived from a locally generated master clock signal whose stability requirement in the GSM system, for instance, is 0.05 ppm. One option to try to meet this stability requirement is to use as the master oscillator of the base station a crystal oscillator positioned in a space having a stable ambient temperature, a so-called oven. A crystal oscillator of this kind is commonly known as an oven stabilized crystal oscillator.
The frequency generated by the oven stabilized crystal oscillator is very accurate in the short term, but due to aging of the oscillator, instability appears in the frequency generated by the oscillator in the long term. When the oscillating frequency changes, the transmission frequency of the base station to be derived from the oscillation of the oscillator changes as well. When the change in frequency grows excessively as compared with the nominal frequency, the transmission frequencies of the base station start interfering with other GSM traffic. A means for improving long-term stability of the oscillator is therefore required.
FIG. 1 illustrates the prior art GSM system. Base stations BTS and a base station controller BSC controlling said base stations constitute a base station system BSS. An interface between a mobile services switching centre MSC and the base station controller BSC is called A-interface and an interface between the base station controller BSC and the base stations BTS is called Abis-interface. FIG. 1 also shows a wireless terminal equipment MS communicating with the base station BTS. Standard, digital PCM (Pulse Code Modulation) connections, e.g. 2.048 Mbit/s, are used in communication between the mobile services switching centre MSC, the base station controller BSC and the base stations BTS. U.S. Pat. No. 5,473,284 discloses a solution in which an adjustable oven stabilized crystal oscillator is used for improving long-term stability of the oven stabilized crystal oscillator of the base station, a reference frequency (e.g. 2.048 MHz) used for the adjustment being derived from the PCM connection between the base station controller and the base station.
Said reference frequency derived from the PCM connection and used for the adjustment of the oven stabilized crystal oscillator can be considered very accurate, since typically the clock signal of the PCM connection is originally synchronized with a signal originating from an atomic clock in the mobile service switching centre MSC.
However, if a PCM clock signal is not available to the base station system, for some reason or other, the solution disclosed in U.S. Pat. No. 5,473,284 is not useful. For instance, this is the case when there is a packet switched network, such as an IP (Internet Protocol) network or an X.25 network, between the mobile services switching centre MSC and the base station system BSS.
A new solution is now found to adjust the oscillator over the packet switched network. A first aspect of the invention implements a method for adjusting the oscillator in a system, the system comprising an adjustable oscillator, a time source knowing a clock time with a specified accuracy, and a packet switched network between the oscillator and said time source.
The method is characterized by
updating a clock on the basis of the time known to said time source over the packet switched network, the clock being on the same side of the packet switched network as the adjustable oscillator;
using said clock in the adjustment of the oscillator for adjusting the oscillator to oscillate at a desired frequency.
A second aspect of the invention implements an apparatus for adjusting the oscillator in a system, the system comprising an adjustable oscillator, a time source knowing a clock time with a specified accuracy, and a packet switched network between the oscillator and said time source.
The apparatus is characterized by comprising
a clock which is on the same side of the packet switched network as the adjustable oscillator;
updating means for updating the clock over the packet switched network on the basis of the time known to said time source;
control means for adjusting the oscillator to oscillate at a desired frequency by means of said clock.
A third aspect of the invention implements computer software for adjusting the oscillator in a system, the system comprising an adjustable oscillator, a time source knowing a clock time with a specified accuracy, and a packet switched network between the oscillator and said time source.
The computer software is characterized by comprising a program code for
updating the clock over the packet switched network on the basis of the clock time known to said time source, the clock being on the same side of the packet switched network as the adjustable oscillator;
using said clock in the adjustment of the oscillator for making the oscillator to oscillate at a desired frequency.
The term digital convergence refers to integrating voice and data communications in a telecommunications network. Typically, it is assumed that the telecommunication network is a packet switched network based on the IP protocol, whereby e.g. transmission of voice communication over said packet switched network is called Voice over IP (VoIP). Network architecture, in which for instance voice and data communications of a company are integrated in the same packet switched network, is called WIO (Wireless Intranet Office). Even though the WIO system and the IP network are used below as examples in a preferred embodiment of the invention, it is also possible to apply the invention to other systems, and the invention can be implemented in essence also in connection with other than IP-based packet switched networks, such as X.25 packet switched network.